Electric machines, such as an electric motor, include a stator and a rotor having opposing surfaces. The stator includes a plurality of slots disposed on a surface of the stator, and a plurality of conducting wires disposed within the slots of the stator. The rotor is rotatable about an axis relative to the stator. The conducting wires generate an electromagnetic force in response to an electric current passing therethrough. The electromagnetic force acts against the rotor to cause the rotor to rotate relative to the stator.
The current density varies throughout the conducting wire based on frequency and due to the presence of magnetic fields. Typically, the majority of the current is conducted through the outer surface region of the conducting wire, commonly referred to as the “skin depth” of the conducting wire, and very little current is conducted through the core, i.e., center region, of the conducting wire. This is commonly referred to as the skin effect of the conducting wire. Additionally, when the conducting wires are disposed adjacent each other and are carrying high alternating currents, each current carrying conducting wire generates a magnetic field in the adjacent conducting wires, which produces a non-uniform distribution of the current in the adjacent conducting wires. This is commonly referred to as the proximity effect.
The conducting wire typically includes either a rectangular cross sectional shape or a circular cross sectional shape. The conducting wires should include a low AC resistance, and should also dissipate heat quickly. The rectangular shaped conducting wires include a high Alternating Current (AC) resistance at high speeds and/or frequencies due to the skin effect and the proximity effect, whereas the small diameter circular shaped conducting wire includes a low AC resistance because the skin effect covers the full cross sectional area of the conducting wire. The circular shaped conducting wire includes difficulty dissipating heat because the circular shaped conducting wire includes a smaller surface area than the rectangular shaped conducting wire, while the rectangular shaped conducting wire has better heat dissipation due to an increased surface area. Furthermore, it is difficult to extract heat from a center of a bundle of circular shaped conducting wires.